1. Field of the Invention
The present invention generally relates to color printing apparatus adapted for printing both sides of a printing paper.
2. Description of the Prior Art
Conventionally, various color printing apparatus for both sides of a printing paper have been well known. One example of such type apparatus has been shown in "Newspaper Technique Information Vol. 20, No. 15" published by Nippon Newspaper Association, dated on Aug. 1, 1988; especially referred to the description from line 1 on page 4 to line 19 on page 10. Such type of apparatus comprises a plurality of printing sections which are isolately arranged in a vertical line a piled state. Each of the printing sections includes a pair of blanket cylinders which can be alternately moved between a contact position and isolated position. The blanket cylinders each have a plate cylinder.
FIG. 6 shows one example of detailed structure of such an apparatus. First, second, third and fourth printing sections 201, 202, 203 and 204 include facing pairs of blanket cylinders 211 and 221; 231 and 241; 251 and 261; and 271 and 281, respectively. The first and second printing sections 201 and 202 are closely arranged, and the third and fourth printing sections 203 and 204 are also closely arranged. Accordingly, the second printing section 202 and the third printing section 203 are arranged apart from each other. On the whole, this layout can shorten the printing length from the first printing section 201 to the fourth printing section 204. Further, plate cylinders 212, 222, 232, 242, 252, 262, 272, and 282, which belong to the printing sections 201, 202, 203, and 204, are respectively provided with plate side adjusting means for adjusting a plate on the plate surface when the plate cylinders are kept in their stop state. The plate side adjusting means is shown in FIG. 8 on page 10 of the above described publication.
The first, second, third and fourth printing sections 201, 202, 203 and 204 include inking units 213 and 223; 233 and 243; 253 and 263; and 273 and 283 and dampening units 214 and 224; 234 and 244; 254 and 264; and 274 and 284, respectively. In the first and third printing sections 201 and 203, the inking units 213 and 223; and 253 and 263 and the dampening units 214 and 224; and 254 and 264 are arranged below the plate cylinders 212 and 222; and 252 and 262, respectively. In the second and fourth printing sections 202 and 204, the inking units 233 and 243; and 273 and 283 and the dampening units 234 and 244; and 274 and 284 are arranged below the plate cylinders 232 and 242; and 272 and 282, respectively.
On the other hand, a printing paper, particularly in a web shape, tends to vary its dimension by moisture. In detail, the dimension of the paper web is sensitive to the ink and the dampening water which are transferred at printing operation. The change in the dimension of the paper web W is remarkably generated in the lateral direction of the paper web because of the wettability of pulp fibers forming the paper web and the orientation of the pulp fibers by paper making. This phenomenon has been well known, and is for example referred to in "Chemistry of Paper" published by Research Association of Chugai Sangyo Inc., 3rd Issue, Mar. 25, 1982.
In a conventional apparatus in which colored ink is applied to both sides of the paper, the distance from the first printing section 201 to the fourth printing section 204 is shortened as much as possible to enable the paper web w to pass through the whole printing sections from the first section 201 to the fourth section 204 within a short period. This arrangement is intended to reduce the influence of moisture upon the printing quality. In other words, the printed pattern by the first printing section 201 deviates more from that printed by the second, third and fourth printing sections 202, 203 and 204 than the central area of the printed pattern because the dimension of the lateral sides of the paper web varies more under the moisture condition than the central area of the paper web. In order to eliminate the deviation as much as possible, the conventional apparatus employs a plate side adjusting means provided at every plate cylinders 212, 222, 232, 242, 252, 262, 272, and 282 for adjusting the plates on the plate cylinders, respectively.
However, conventional plate side adjusting means can not be operated while the plate cylinders 212, 222, 232, 242, 252, 262, 272, and 282 are revolving. To adjust the position of the plates on the plate cylinders, the cylinders should be completely stopped. This adjusting work requires skilled operators.
Further, this adjusting system will cause extremely poor operation efficiency if each rolled paper web is exactly corrected in response to the variation in the dimension depending on each web so that this adjusting operation has been practically employed.
In conventional printing apparatus as shown in FIG. 6, while the dampening units 214, 224, 254 and 264, arranged below the plate cylinders 212, 222, 252 and 262 of the first and third printing sections are disposed inwardly of the inking units 213, 223, 253 and 263, the dampening units 234, 244, 274 and 284, arranged above the plate cylinders 232, 242, 272 and 282 of the second and fourth printing sections, are disposed outwardly of than the inking units 233, 243, 273 and 283. These arrangements are remarkably complicated so that operation efficiency will become poor and operation mistakes will occurs often. Further, these arrangements may cause various problems in view of design, manufacturing, maintenance, and so on.
In fluid supplying systems such as the dampening units and the inking units, the fluid flowing in a downward-direction tends to feed a greater amount than that of the fluid flowing in the upward-direction owing to the influence of gravity upon the fluid flow. Thus, the dampening units 234, 244, 274 and 284; and the inking units 233, 243, 273 and 283, arranged above the plate cylinders 232, 242, 272 and 282, and it is more difficult to finely control their feeding amount than that of the dampening units 214, 224, 254 and 264; and the inking units 213, 223, 253 and 263 arranged below the plate cylinders 212, 222, 252 and 262. This control requires extremely skillful operators. Further, the dampening units 234, 244, 274 and 284; and the inking units 233, 243, 273 and 283 arranged above the plate cylinders 232, 242, 272 and 282, requires means for preventing excess feeding of dampening water and ink. For example, the downstream rollers should be arranged higher than the upperstream rollers. This further requires space for roller arrangement, additional consideration in design, and so on. In addition to these disadvantages, this requirement is an obstruction to the minimization of the roller numbers and to the realization of a compact body of the printing apparatus.
Although a color printing operation always requires a complete agreement of each color printed pattern, conventional both sides color printing apparatus can not completely perform such pattern agreement because the distance between the first printing section 201 and the second printing section 202 and between the third printing section 203 and the fourth printing section 204 is different from the distance between the second printing section 202 and the third printing section 203. More specifically, the circumferential phase of the plate cylinders 212 and 222 associated with the first printing section is difficult to align that of the plate cylinders 232, 242, 252, 262, 272 and 282 associated with the second, third and fourth printing sections 202, 203, 204. Thus, the circumferential phases of the plate cylinders 212, 222, 232, 242, 252, 262, 272 and 282 are firstly adjusted to be substantially the same, and then the respective plates are simultaneously mounted on them. This simultaneous mounting operation results in a complicated configuration. If such a complicated configuration is not completely designed, the simultaneous mounting operation will not result and therefore this will cause a poor efficiency in the printing operation.